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Two engineering recommendations are currently in use by the UK DNO’s, Termed G83 and G59.
Explanation of each recommendation.
Certain areas of the UK fall under different requirements and stipulations in relation to connection. An important and confusing point to note at this stage is the DNO’s class the inverter as the generator and not the power producing device. Therefore, when connection of a parallel device is considered, it is the inverter to which is referred.
If an area of the country is G83 only connections, a parallel device cannot be grid tied unless it is G83 certificated. G83 restricts export to the grid to a maximum of 16 amps per phase or 3.68kW on single phase and approx. 11kW on three phase. If a larger power source is connected via a G83 inverter, example 5kWp of solar on single phase, the inverter is unable to produce more than 3.68kW output, it will therefore, produce the maximum for longer, but never the peak production possible. It is worth noting also at this point, that without an alternating current, ie a grid, for the inverter to “recognise” it will not operate. A second point worth noting, is DNO’s are obliged to allow connection via a G83 certificated generator and the process is one of notification of parallel generation rather than formal application.
It is widely believed that larger scale renewables can be connected in G83 areas using a G59 inverter and export control devices such as EMMA. A system of this type can control export to within G83 parameters and requirements by diverting excess power to loads, which are activated as power above the export limit and site demand becomes available. If we refer back to para 2 though, we see that this is not a connection using a G83 certificated generator, but connection using a G59 generator with export restricted. This is not allowable in a G83 constrained area. The areas with such DNO stipulations are growing throughout the UK with much of Scotland and the South governed by the recommendation as the high voltage network reaches a point of maximum capacity.
In such areas it is normally not possible to upgrade the grid by the generator owner to accept lager scale installations, as these would again fall outside of G83 recommendations.
Engineering Recommendation G59 allows connection of larger than 16 amps per phase parallel generation, subject to certain conditions being met at at the location of installation. These can include grid upgrade to increase network capacity locally in the area, export control, as mentioned in the previous paragraph, or possibly, if the grid is currently robust enough, no cost connection.
Export Control Further Considerations
Should the installation site be in a G59 area of the UK, and export restriction such as EMMA Export Control is permitted by the DNO as part of the installation, then the diverted power must be put to genuine and tangible use. The loads that will be switched on as excess power becomes available will generally be resistive by nature, for example an immersion heater element/s raising the temperature of water in a thermal store for use as domestic hot water, farm wash down and sterilisation or central heating. An example of non allowable diversion would be to an electric patio heater. If the diversion is not an allowable load then the installation will not qualify for FiT subsidy.
Alternative Methods of Connection.
Clearly from the above, there are a number of challenges to overcome when connecting parallel generation to the nation grid if the installation is in excess of 3.68kW per phase. It is however possible to connect using methods other than listed above.
Provided there is sufficient demand on site, connection in a G59 stipulated area can be made by placing a G59 relay between the generator (inverter) and the national grid. This method is widely accepted by the DNO as an alternative method of connection and does not require energy diversion devices or grid upgrades. The relay is set to operate and shut down generation if certain export parameters are exceeded. The installation and operation of the relay is also witness tested by the DNO, so as to guarantee compliance, prior to commissioning. The advantages are the non-requirement for expensive upgrade and normally a more acceptable method of connection. The cost of the relay, witness testing and fitting, should be balanced against the cost of possible upgrades, and the client must be certain there is sufficient demand on site to meet the expected generation, otherwise the relay will shut down the system due to over export resulting in a loss of FiT.
Large scale renewable energy grid connection in grid constrained areas.
Now possible without expensive grid upgrades or outright refusal to parallel
CPSL has developed a patented method of grid connection for larger scale generation by using a G83 and a hybrid arrangement, whereby the export is contained within the 16 amp per phase requirements but site demand is met from renewable sources. The system also includes some battery storage sized to support site demand at times of little or no renewable production. The system is in the final phase of development and testing, with orders expected to be accepted from quarter 1 2018
G59 Connection Using Battery Storage to Assist Site Demand.
There are many systems of battery storage for use with renewable energy production, mainly though small scale in nature, running at 48v and high current. An installation of this type can result in costly additional components and cabling. A spin off device of the system in the previous paragraph will allow larger scale storage at higher voltages. This will be the next stage of the Smartpower development. Smartpower can however now, install 48v storage systems, with limited maximum support at one time. The system can be in 3 phase or single phase and has a number of successful installations already undertaken.
Battery Storage Consideration When Sizing a System.
There are two approaches which may be taken in order to gather the necessary information to create a pattern of use for a particular site. If we first look at the information required, we can then examine how accurate data can be obtained.
Annual total use of electricity (kWh from previous invoices)
Quarterly use of electricity (kWh from previous invoices)
Number of Circuits to site.
Use of Circuits (eg Upstairs sockets, Cooker, Milking Parlour etc.)
Appliances present on each circuit, particularly and unusual high peak demands, often caused by inductive loads undergoing a start-up procedure, such as a compressor, welder etc.
Are loads such as in number 5 used at times of low or no production and power outages.
From this information, we can begin to develop a basis for a system design, sizing battery capacities, battery types and charge and discharge rates.
Alternatively, monitoring equipment can be installed for a two week period to establish site patterns and usage over a given period. This will provide a “snapshot” but obviously not an assessment of annual or seasonal site use. This method may be adopted if the site owner advises patterns of use are usually fairly constant.
There are at present a number of new battery technologies emerging for use in the renewable sector. These include, but are not limited to:
Although traditionally lead acid is the most recognised method of battery storage, other forms of storage fit better to the charge and discharge patterns of renewable storage and site support. We should consider a lead acid battery is generally designed to provide a short, high current burst of energy to start a vehicle. If discharged below 50% of it’s capacity, the life of the battery will be shortened considerably. If NiCad or Lithium Ion batteries are utilised, they can be discharged to empty and recharged to full many times, consequently, although more expensive initially, only 50% of the capacity is required to provide the same discharge characteristics. Flow Batteries and Salt Batteries are still in a development stage.